IONIC:如何访问assets文件夹中的文件？

import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class FileService {

  constructor(private http: HttpClient) { }

  getFileContent(filePath: string): Observable<string> {
    return this.http.get(filePath, { responseType: 'text' });
  }
}

import { Component } from '@angular/core';
import { FileService } from 'path-to-file-service';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.scss'],
})
export class ExampleComponent {

  constructor(private fileService: FileService) { }

  getFileContent() {
    const filePath = 'assets/documents/example.txt';
    this.fileService.getFileContent(filePath).subscribe(content => {
      console.log(content);
    });
  }
}